Method and apparatus for the calibration of a multichannel printer

ABSTRACT

A method and apparatus are disclosed for the calibration of a multichannel printer. The printer comprises a plurality of diode lasers which are modulated in accordance with an information signal. Radiation from the diode lasers is imaged onto a receiving medium such as a thermal print medium. The thermal print medium is supported on a drum which is rotatable about an axis. In order to balance the output from the diode lasers, test patterns are formed on the receiving medium with each of the lasers using known power inputs. The density of each of the test patterns is measured, and the density values obtained are correlated with the input power levels for each of the lasers. The measurement of densities produced by a single laser is facilitated by slowing the print head down by a factor of the number of lines normally printed and writing with one line source at a time.

BACKGROUND OF THE INVENTION

1. Cross-Reference to Related Applications

Reference is made to U.S. patent application Ser. No. 724,059, entitled"Method and Apparatus for Scanning a Receiving Medium," filed on Jul. 1,1991, in the names of S. H. Baek et al.; Ser. No. 724,061, entitled"Method and Apparatus for Scanning a Receiving Medium," filed on Jul. 1,1991, in the names of Baek, Mackin, Firth, and Woo; and Ser. No.724,060, entitled "Method and Apparatus for Scanning a ReceivingMedium," filed on Jul. 1, 1991, in the names of Mackin et al. Referenceis also made to U.S. patent application Ser. No. 749,031, entitled "AMethod of Calibrating a Multichannel Printer," filed on even dateherewith in the name of Sanger et al. All of these applications areassigned to the assignee of the present invention.

2. Field of the Invention

The present invention relates to the calibration of a multichannelprinter, and more particularly, to the calibration of such a printer inorder to eliminate artifacts in an image produced thereby.

3. State of the Prior Art

In one type of scanner apparatus, a photosensitive material is supportedon a rotatable drum, and a print head carrying a light source isadvanced relative to the photosensitive material by means of a leadscrew. The light source is modulated in accordance with an informationsignal to form an image on the photosensitive material. In order toincrease the output of such apparatus, multiple light sources aremounted in the print head so that a plurality of print lines can beformed in a single pass. In multiline scanning systems, any differenceof densities among the lines can create very severe artifacts in theimage. These artifacts can appear as repetitive patterns known as"banding."

The problem of banding can be particularly troublesome in half-toneprinting where, for example, 12 mini-pixels are used to write ahalf-tone dot. Visible lines in the image, caused by unevenness in thedensities of the lines, can come at a different section of eachsuccessive half-tone dot, and thus, cycle across the image. The visiblelines can be due, for example, to a variation in the intensity of thelight sources. The frequency of the visible lines in the image beatswith the half-tone dot frequency. The resulting macro density variationcan have a spatial frequency in the image which, unfortunately, matchesthe frequency at which the eye is most sensitive, that is, at about 0.5cycle/mm. At this frequency range, the typical human eye can see avariation of density of around 0.2%. This small level of unevenness indensity is very hard to control in a printer using a multiline printhead.

The aforementioned U.S. Patent Applications are directed to solutions tothe problem of artifacts in images created by printers. In certain ofthe applications, random noise is introduced into the control signals ofthe printer in order to mask artifacts in an image produced by theprinter. Other techniques have been used in the art to overcome theproblems of image artifacts. For example, certain multiline film writersare capable of printing half-tone images, without artifacts, by using ahigh gamma film under saturated exposure conditions. Another techniquewhich is used to mask artifacts is to vary the resolution of the filmwriter. Changing the line-to-line pitch and the line width of the filmwriter permit a selection of resolution which will limit artifacts atthe desired half-tone ruling and screen angle. A disadvantage ofincreasing resolution is that it also increases both the amount of datarequired and the time to make each image.

Multiple writing beams can be formed from a single light single sourceby using beam splitter optics. Such a system will typically write sixlines at a time. In these systems, the problem of balancing each writingline may be less severe, since the same light source is used for all ofthe lines; however, the intensity of light provided to each line isnecessarily considerably reduced. None of the known prior-art solutionsto the problem of artifacts in images produced by multiline printers istotally satisfactory, however, particularly when the writing is beingdone on mediums such as a thermal print medium.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the problems in theprior art discussed above and to provide an improved method andapparatus for calibrating a multichannel printer.

In accordance with one aspect of the present invention, there isprovided a method of calibrating a multichannel printer, the printerhaving a plurality of light sources which are projected onto a receivingmedium responsive to radiation, the method comprising the steps of:sequentially directing radiation from each of the light sources onto areceiving medium to form a test pattern thereon, the test patternincluding a plurality of test patches, each of the test patches beingformed by radiation from one of the light sources produced by a knowninput; measuring the density of each of the test patches and correlatingthe input of each of the light sources with the measured density of therespective test patch; and determining the inputs for each of thesources to obtain desired densities on said receiving medium.

In accordance with another aspect of the present invention, there isprovided apparatus for calibrating a multichannel printer, the printerhaving a print head which includes a plurality of light sources, theapparatus comprising: means for directing radiation from the print headonto a receiving medium responsive to radiation in order to form aplurality of lines thereon; and means for controlling the directingmeans to form a plurality of test patches on the receiving medium, eachof the test patches being formed by radiation from one of the lightsources.

The present invention can be used in a printer which is adapted to forman image on a thermal print medium. The printer includes a rotatabledrum for supporting the print medium, and a print head which is movedrelative to the print medium by means of a lead screw. The print headincludes an array of optical fibers and a lens for focussing the ends ofthe fibers onto the medium. Each of the fibers is connected to a diodelaser which is modulated in accordance with an information signal. Inorder to balance the output from the diode lasers, the power output fromeach of the lasers is measured, and the power measurements arecorrelated with the power inputs to the lasers. Test patterns are thenformed on the receiving medium with each of the lasers using known powerinputs. The density of each of the test patterns is measured, and thedensity values obtained are correlated with the input levels for each ofthe lasers. The measurement of single laser densities is facilitated byslowing the print head down by a factor of the number of lines normallyprinted and writing with one line source at a time.

A principal advantage of the present invention is that apparatuscalibrated in accordance with the invention can produce an image free ofartifacts in both half-tone and continuous-tone printing. A furtheradvantage is that increased resolution can be obtained in both modes ofoperation. A still further advantage of the present invention is that animage free of artifacts can be produced without the need to modify theprinter apparatus.

Other features and advantages will become apparent with reference to thefollowing description of the preferred embodiment when read in light ofthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a multichannel laser thermal printer;

FIG. 2 is a schematic view of the drum and print head of themultichannel printer;

FIG. 3 is a diagram showing the data path of the multichannel printer;

FIG. 4 is a schematic view of a channel driver;

FIG. 5 shows a test pattern generated in accordance with the presentinvention;

FIG. 6 shows a continuous-tone test pattern generated in accordance withthe present invention; and

FIG. 7 shows a test pattern generated when two adjacent channels areenergized at one time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be used in a multichannel printer such aslaser thermal printer 10 shown in FIG. 1. Printer 10 comprises a drum 12which is mounted for rotation about an axis 15 and is driven by a motor14. Drum 12 is adapted to support a thermal print medium, not shown, ofa type in which a dye is transferred by sublimation from a donor to areceiver as a result of heating the dye in the donor. A thermal printmedium for use with the printer 10 can be, for example, a mediumdisclosed in U.S. Pat. No. 4,772,582, entitled "Spacer Bead Layer forDye-Donor Element Used in Laser Induced Thermal Dye Transfer", grantedSep. 20, 1988. This patent is assigned to the assignee of the presentinvention.

A print head 20 is movably supported adjacent drum 12. Print head 20 issupported for slidable movement on a rail 22, and the print head 20 isdriven by means of a motor 24 which rotates a lead screw 26. Print head20 comprises an array of optical fibers 31 which are connected to aplurality of light sources such as diode lasers (not shown). The diodelasers can be individually modulated in a well-known manner toselectively direct light from the optical fibers onto the thermal printmedium. A more complete description of the thermal printer describedherein can be found in U.S. application Ser. No. 451,655, filed on Dec.18, 1989, in the names of Seung H. Baek et al., and the disclosure inthis application is expressly incorporated herein by reference.

In multichannel printers, any difference in the density of the printlines creates a very severe artifact known as banding. This artifact canbe due, for example, to unbalanced laser power, and the artifact isparticularly troublesome in half-tone printers. In half-tone printers,it takes, for example, 12 mini-pixels to write a half-tone dot. Theprinting swath may sometimes be less than 12 mini-pixels, and as aresult, a darker line (or several darker lines) comes at a differentsection of each sequential half-tone dot; these darker lines cycleacross the image. The frequency of the lines of uneven density beatswith the half-tone dot frequency and is visually observed in the imageas banding. The resulting macro density variation can have a spatialfrequency in the image which matches with the greatest contrastsensitivity of the eye.

In the present invention, the light source in each of the channels isprecisely calibrated in order to overcome the problem of banding. Atarget image is formed with each light source independently. Densitytransmittance measurements on the target images are used to balance eachlaser. Power measurements can be made periodically during the operationof the instrument to track the performance of each light source.

The measurement of single-line print densities is facilitated by slowingthe lead screw down by a factor of the number of lines normally printed,and writing with one line source at a time. This produces an image madeusing a single channel at the normal line-to-line spacing. The printdensity is measured with a conventional densitometer. Single linedensities are referred to herein as single laser densities or (SLD's). Asingle writing channel or single line is referred to as a single laser.

Measuring print densities on a line-by-line basis provides a means toaccount for differences in the output of the individual lasers. Thismethod of balancing does not require that all of the individual factorsaffecting laser output be measured or even that all of the significantcontributors be known. Once the lasers have been balanced, routine powermeasurements can be made to track the performance of the writing engineover the lifetime of the machine. This is a simpler and less timeconsuming method than repeatably printing single line densities to trackeach laser over time.

In one embodiment of the present invention, the following steps areperformed: (1) single laser density (SLD) target images are printed on areceiving medium using known inputs (DAC values) for each laser; (2) thetarget images are then measured with a densitometer, and the measuredSLD's are correlated with the DAC values for each source; (3) DAC valuesneeded to obtain a given single laser density are then computed, usingthe SLD versus DAC value relationship calculated in step 2; (4) solidarea densities (SAD's) are printed using all of the lasers in the printhead 20 to print selected sets of SLD's. For a desired SLD in this step,each laser would be set to the DAC value which has been found to producethat SLD; and (5) a regression is calculated between the SAD values andSLD values. As a result of the foregoing steps, a given SAD can beproduced by using the corresponding SLD found in step 3.

The above steps can be repeated as needed to maintain the balance amongthe separate channels in printer 10. However, it is preferable toperform power measurements in order to maintain balance, once therelationship between SLD and DAC values is defined for each channel.Thus, to maintain balance of the channels over time using powermeasurements, the following six steps are performed: (1) the power foreach laser versus the input (DAC value) is measured; (2) single laserdensity (SLD) target images are printed using known power levels foreach laser; (3) the target images are then measured with a densitometerand the density values obtained are correlated with the input values foreach source; (4) the power of each laser is remeasured when rebalance ofthe system is found to be necessary; (5) DAC Values to obtain a givenSLD are readjusted using the latest power data measured in step 4, andthe original single laser density versus power relationship calculatedin step 3; and (6) solid area densities (SAD's) are printed using all ofthe lasers in print head 20 to print selected sets of single laserdensities. A regression is then calculated between SAD values and SLDvalues. Thus, when a particular SAD is desired, the corresponding SLDcan readily be determined.

Performance of the steps described in the preceding paragraph will nowbe described with reference to FIGS. 1 and 2. The single laser power atthe revolving drum 12 is proportional to the DAC value for that laser.In step 1, the print head 20 is moved to a calibration station 34, shownin FIG. 2. The analog output of a calibration sensor (not shown) atcalibration station 34 is fed into an analog-to-digital converter (notshown). The calibration sensor can be, for example, a SD 444-4141-261sensor, obtainable from Silicon Detector Corp. Analog-to-digitalconverter counts (ADC values) are proportional to the power detected bythe calibration sensor, and ADC values for a set of DAC values aremeasured for each laser. A linear regression is performed on the datafor each channel to model the power (ADC Value) as a function of DACvalue. Equation 1, shown below, is the model used.

    (ADC Value)=m1 * (DAC Value)+b1                            (1)

    (DAC Value)=[(ADC Value)-b1]/m1                            (2)

where m1 is the slope and b1 is the intercept.

In step 2, a set of ADC values are selected which will produce workablesingle laser densities (SLD's). The DAC values required to obtain thisset of ADC values are calculated using Equation 2. Single laserdensities are printed by turning on one channel at a time and slowingthe speed of lead screw 26 down to one line width per revolution of thedrum 12.

Normally, lead screw 26 is driven at a speed which will translate theprint head 20 a distance equal to the width of the number of lineswritten during one revolution of the drum 12. For example; if thespacing of the lines is 100 lines per mm. and the number of writinglines is ten, the print head 20 would be moved, as follows. ##EQU1##

During single laser tests the print head will be slowed down to:##EQU2##

The resulting test image when printed using a single laser source willhave the same line spacing as a normal image formed with a print headhaving ten lasers.

In step 3, the single laser densities are measured using an ordinarydensitometer, for example, a X-Rite 408G densitometer. The resultingsingle laser densities are plotted against the known ADC values. Asimple linear regression is performed to calculate the slope andintercept for the model shown in the following equation for eachchannel.

    SLD=m2 * (ADC Value)+b2                                    (3)

In Step 4 the print head is again moved to the calibration station 34.ADC counts for a set of DAC values are remeasured for each laser. Alinear regression is performed on the data for each laser channel tomodel the power (ADC Value) as a function of DAC value. Equation 4,shown below, is the model used. Step 4 is repeated as needed to trackthe performance of each laser over the life of printer 12.

    (New ADC Value)=m3 * (DAC Value)+b3                        (4)

In Step 5 the slopes and intercepts calculated in the previous steps areused to calculate the DAC Value that will result in a required singlelaser density. The following equation is used to calculate the requiredDAC value to obtain a requested SLD.

    DAC Value Required={m1*[(SLD-b2)/m2]+b1-b3}/m3             (5)

The final step, Step 6, is directed to printing solid area densities(SAD's) using all of the lasers to print selected sets of balancedsingle laser densities (SLD's). A linear regression is performed torelate SAD values to SLD values using the model in the followingequation:

    SAD=m4*SLD+b4                                              (6)

Equation 7, shown below, is used to calculate the required single laserdensity when it is desired to produce a print at a particular solid areadensity. Equation 5 is then used to determine the DAC value to be usedfor each laser channel.

    SLD=(SAD-b4)/m4                                            (7)

In the calibration of a multichannel printer according to the presentinvention, individual laser channels can be balanced without measuringor knowing all of the factors which cause the densities produced by thedifferent channels in the printer to be different. Some of the factorswhich may contribute to differences in density are: the spot size ofeach channel, the wavelength of each laser source, and the modes thateach source emits through the print head. Since these factors can varyfrom laser to laser, the densities produced by the lasers can bedifferent even when the power output from the lasers is the same. Thus,the correlation of the SLD's with the measured power outputs of thelasers is an important feature of the invention

The ability to eliminate artifacts by precisely balancing each channelpermits an increase in the number of lines printed at one time, therebydecreasing the total time required to make a print. By using powermeasurements to track subtle changes in laser channel performance,measurement of the single laser densities only needs to be done once. Asa result of using the technique of slowing the lead screw 26 to a rateof one line per revolution of the drum, complex microdensitometry is notrequired in the calibration process.

With reference to FIGS. 3-6, additional methods will now be describedfor obtaining the desired test areas from each of the lasers in printhead 20. In one method, a test pattern is created which includes aplurality of test patches formed during the revolution of the drum 12.Each test patch is created using a different laser. This method makes itpossible to print all n lasers during the same amount of time that themethod described above prints one laser. Another method uses each laserto print test patches while the drum rotates, each test patch beingexposed at a different power level.

In the schematic drawing of multichannel printer 10 shown in FIG. 2,print head 20, containing n channels, is moved relative to revolvingdrum 12 which supports a thermal print medium (not shown). In the normaloperation of printer 10, the print head 20 moves a distance equal to nlines for each revolution of drum 12. Modulated data is sent to each ofthe n channels to expose the media in accordance with an informationsignal in order to create an image on the medium. The power level ofeach channel is adjusted to produce a desired density.

The data path for printer 20 is shown in FIG. 3. A data interface module40, (DIM), presorts the Image Data and sends it to the appropriatechannel driver 42. For example, if the image data is sent one line at atime, the data interface module 40 will transfer the first line tochannel 1, the second line to channel 2, the n^(th) line to channel n,and the n^(th) +1 line to channel 1. The channel drivers 42 buffer anddeliver the data to each channel in the print head 20 at the correcttime. The channel drivers 42 also convert the data from an on or offsignal to optical on and off power levels. A more detailed drawing of achannel driver 42 shown in FIG. 4.

The data interface module 40 loads 8 bits of data at a time into channeldriver 42. In a half-tone printing mode, this data represents 8 bitseach of which is either on or off. The 8 bits are stored in afirst-in-first-out (FIFO) buffer 46 until all the channels are ready andthe drum 12 is up to speed. The 8 bits are then loaded into a shiftregister 48 and shifted out 1 bit at a time to the lowest address line,(a0), of a look up table 50. The look up table 50 is loaded such thatwhen a0 is a logic low the 12 bit output value is set to the off powerlevel and when a0 is a logic high the 12 bit output value is set to theon power level. The 12 bit DAC Value out of the look up table drives adigital-to-analog converter (not shown). The analog output is fed into acurrent amplifier (not shown) which drives the laser of one channel.

The channel driver 42 can, alternatively, be configured forcontinuous-tone printing instead of half-tone printing. Incontinuous-tone printing each byte, 8 bits, of data sent from the DIM 40is used to select one of 256 power levels. The shift register 48 is notused, and the data out of FIFO buffer 46 is fed directly into the lookup table 50. The look up table 50 is loaded with values which convertthe 8 bit input value into 12 bit power levels which result in equalincrements in print density.

The channel driver 42 may also be configured to use the calibration pathinstead of the normal print path. In this configuration, the DAC Valueinto the digital-to-analog convertor is driven by the calibrationregister 52 instead of the look up table 50.

During a single laser test, the drive motor 14 for lead screw 26 isdriven at 1/n times normal speed. Thus, the print head 20 moves exactlyone line width for each revolution of the drum 12. The channel drivers42 are configured for half-tone printing. A constant solid image is fedinto the DIM 40. Only one channel driver 42 out of n is turned on at atime. Data sent from the DIM 40 to the unused channels is ignored. Theresulting image contains lines at the correct line spacing which wereexposed using only one channel. Single laser tests using this method mayalso be created by: (1) turning the drum 12 at the normal speed; (2)turning the lead screw at 1/n times the normal speed; and (3) turning onone laser at the desired power level, setting its DAC Value by means ofthe calibration register 52.

A more efficient method of generating single laser densities is to printa test pattern 60 as shown in FIG. 5. The test pattern 60 repeats aftern lines. Lines 1 through n contain x pixels to be printed in a sequencesuch that if the lines were placed on top of each other their exposedareas would not overlap. When printed at a lead screw speed which movesthe printhead one line per revolution of the drum 12 the resulting imagewill contain n patches 62. Each patch 62 will have been exposed by aseparate channel. The test pattern 60 can be generated using an internalpattern generator (not shown), or the necessary instructions can existas a special line work image file.

In FIG. 5, there is shown a test pattern 60 generated by a print headhaving 9 channels. Each channel forms one line 64 so n is 9, and eachline contains 10 pixels so x is 10. The first line has 10 pixels on, andall other pixels off. No other line in the pattern is on at the sametime as line 1. Line n+1 is the same as line 1. Therefore; channel 1will print during lines 1, n+1, 2n+1, etc. and none of the otherchannels will be active at the same time. The first 10 pixels in theresultant image will be exposed only by channel 1. The second 10 pixelsare exposed only by channel 2, and the last 10 pixels are exposed bychannel 9.

The test pattern 60 can be generated by a pattern generator (not shown)internal to the data interface module 40. The pattern generator cancontain up to 2047 words in the pattern. For a system with 18 writingchannels, for example, the pattern repeats every 18 lines. Each line is1800 pixels or 225 bytes long. Each line contains 100 pixels that areexposed. The first line contains 100 pixels on, and 1700 pixels off. Thesecond line has 100 pixels off, 100 pixels on, and 1600 pixels off. Thissequence continues until the 18th line which contains 1700 pixels offand 100 pixels on. The total pattern length is 2025 words. At aresolution of 1800 pixels per inch the pattern generated would be only 1inch long. Each patch 64 would be only 0.056 inches long making itdifficult to measure with a densitometer. By changing the resolution inthe pixel direction to 300 dots per inch, the test pattern 60 isstretched to 6 inches long. Each patch becomes 0.33 inches long, andsuch a patch can be measured by a densitometer using, for example, a0.25 inch diameter aperture.

The continuous-tone data path can also be used to create test patches.With reference to FIG. 6, there is shown a continuous-tone pattern 70which includes test patches 71-75 made at different exposure levels.Patches 71-75 were made at exposure levels of 255, 223, 191, 63, and 31look up table power levels, respectively. Initially, the look up table50 is loaded with a linear table so that input power level 255 resultsin output DAC value 4095 and input power level 0 results in an outputDAC value 0. Each of the patches 71-75 was printed with one light sourceand with the lead screw 26 running at 1/n times the normal rate. In thecontinuous tone mode, each pixel in the image is sent by the datainterface module 40 to the channel drivers 41. The channel drivers 42pass the 8 bits of pixel data directly to the look up table 50 whichsets the DAC Value, thereby controlling the exposure level of eachpatch. The data sent from the DIM 40 to the unused channels is ignored.The resulting image will contain test patches formed in the direction ofthe movement of drum 12.

From the foregoing description of the present invention, it will be seenthat by slowing the lead screw velocity to one line width per revolutionof the drum the density of a single writing channel out of a multiplenumber of writing channels can be measured directly using an ordinarydensitometer. Test patterns can be created for any multichannel printer,and more than one source can be measured during a single revolution ofthe drum. On a multichannel continuous-tone printer, a single channelcan be used to form multiple patches of different exposures during onerevolution of the drum.

The method implemented herein to print single laser densities can beoptimized in accordance with the total number of channels in a printerand the number of exposure levels required. Test patterns can be createdto efficiently test exposure from a multiple number of individualsources. The test pattern in FIG. 5 could, for example, be modified tochange the order in which each source is printed. Other test patternscan be created which allow for exposure as a function of a multiplenumber of channels. For example, a test pattern 80, shown in FIG. 7, wasproduced by running the lead screw speed at 2/n times the normal speed.Each test patch 82 in the test pattern 80 is the result of exposure fromtwo adjacent channels simultaneously. Such a test pattern can be used toprovide information on channel interaction.

The invention has been described in detail with particular reference tothe preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:
 1. A method of printing test patterns with a multichannelprinter, said printer having a plurality of "n" of light sources whichare projected onto a receiving medium responsive to radiation wherein"n" is an integer, said receiving medium being supported on a rotatabledrum and said light sources being mounted on a translator which isdriven axially of the drum, said method comprising the stepsof:sequentially directing radiation from each of said light sources ontosaid receiving medium to form a test pattern thereon, said test patternincluding a plurality of test patches, each of said test patches beingformed by radiation from a set of said light sources produced by a knowninput, said set of light sources including one or more "a" adjacentlight sources and wherein "a" is a whole integer; and driving saidtranslator at a speed of a/n of the speed which would form successivesets of "n" scan lines.
 2. A method, as defined in claim 1, wherein saidtest patches are formed by actuating each of said light sources insequence for a predetermined number of pixels.
 3. A method, as definedin claim 2, wherein each of said test patches is formed by operatingsaid printer in a half-tone mode.
 4. A method, as defined in claim 2,wherein each of said test patches is formed by operating said printer ina continuous-tone mode.
 5. A method, as defined in claim 2, wherein testpatches are formed by operating each of said light sources at differentpower settings.
 6. A method, as defined in claim 1, wherein each of saidtest patches is formed by a plurality of light sources.
 7. A method, asdefined in claim 1, wherein said receiving medium is a thermal printmedium.
 8. Apparatus for printing test patterns with a multichannelprinter, said printer having a print head which includes means forimaging radiation from a plurality of "n" of lasers onto a receivingmedium which is responsive to radiation wherein "n" is an integer, saidapparatus comprising:means for supporting said receiving medium; meansfor effecting relative movement between said print head and saidreceiving medium in a scan direction and a cross scan direction; andmeans for sequentially directing radiation from each of said lasers ontosaid receiving medium to form a test pattern thereon, said test patternincluding a plurality of test patches, each of said test patches beingformed by the radiation from a set of said lasers, said set of saidlasers including one or more "a" adjacent lasers and wherein "a" is aninteger; and means for driving said relative movement means in a crossscan direction at a speed of a/n of the speed which would formsuccessive sets of "n" scan lines.
 9. Apparatus, as defined in claim 8,wherein said apparatus includes means for measuring the power output ofeach of said lasers.
 10. Apparatus, as defined in claim 8, wherein saidreceiving medium is a thermal medium.